Photovoltaic device

ABSTRACT

A photovoltaic cell to convert low energy photons is described, consisting of a p-i-n diode with a strain-balanced multi-quantum-well system incorporated in the intrinsic region. The bandgap of the quantum wells is lower than that of the lattice-matched material, while the barriers have a much higher bandgap. Hence the absorption can be extended to longer wavelengths, while maintaining a low dark current as a result of the higher barriers. This leads to greatly improved conversion efficiencies, particularly for low energy photons from low temperature sources. This can be achieved by strain-balancing the quantum wells and barriers, where each individual layer is below the critical thickness and the strain is compensated by quantum wells and barriers being strained in opposite directions minimizing the stress. The absorption can be further extended to longer wavelengths by introducing a strain-relaxed layer (virtual substrate) between the substrate and the active cell.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an improved photovoltaic device/cellfor the conversion of heat radiation into electricity.

[0003] 2. Description of the Prior Art

[0004] Thermophotovoltaics (TPV) is the use of photovoltaic (PV) cellsto convert heat radiation, e.g. from the combustion of fossil fuels orbiomass, into electricity. The energy spectrum is often reshaped usingselective emitters which absorb the heat and re-emit in a narrow band.The re-emitted radiation may be efficiently converted to electric powerusing a PV cell of appropriate low band-gap. Higher PV cell efficienciescan be achieved by introducing multi-quantum-wells (MQW) into theintrinsic region of a p-i-n diode if the gain in short-circuit currentexceeds the loss in open-circuit voltage [K. W. J. Bartham and G.Duggan, J. Appl. Phys. 67, 3490 (1990). K. Barnham et al., AppliedSurface Science 113/114, 722 (1997). K. Barnham, International PublishedPatent Application WO-A-93/08606 and U.S. Pat. No. 5,496,415 (1993)]. AQuantum Well Cell (QWC) in the quaternary system InGaAsP lattice-matchedto InP substrates is a promising candidate for TPV applications as theeffective band-gap can be tuned, out to about 1.65 μm(In_(0.53)Ga_(0.47)As), without introducing strain, by varying the welldepth and width, to match a given spectrum. The enhancement in outputvoltage of a QWC is a major advantage for TPV applications [P. Griffinet al., Solar Energy Materials and Solar Cells 50, 213 (1998). C. Rohret al., in Thermophotovoltaic Generation of Electricity: Fourth NRELConf., Vol.460 of AIP Conf. Proc. (American Institute of Physics,Woodbury, N.Y., 1999), pp.83-92].

[0005] There is considerable interest in extending the absorption tolonger wavelengths for higher overall system efficiencies with lowertemperature sources; and lower temperature fossil sources have alsolower levels of pollution. Appropriate and inexpensive substrates of therequired lattice constant and band-gap are not available, so the lowerband-gap material is often strained to the substrate, introducingdislocations which increase non-radiative recombination. Freundlich etal. have proposed strained quantum well devices [U.S. Pat. No. 5,851,310(1998), U.S. Pat. No. 6,150,604 (2000)], but these can only incorporatea restricted number of wells without creating dislocations. Freundlichproposes limiting the number of wells to a maximum of 20, which will notproduce sufficient absorption for efficient generation however. In a MQWsystem, these dislocations can be reduced by strain-balancing thelayers; alternating barriers and wells have bigger and smallerlattice-constants, but on average are lattice-matched to the substrate[N. J. Ekins-Daukes et al., Appl.Phys.Lett.75, 4195 (1999)].

SUMMARY OF THE INVENTION

[0006] Viewed from one aspect the invention provides a photovoltaicdevice having a multiple quantum well portion with alternating tensilestrained layers and compressively strained layers, said tensile strainedlayers and said compressively strained layers having compositions suchthat a period of one tensile strained layer and one compressivelystrained layer exerts substantially no shear force on a neighbouringstructure.

[0007] The invention recognises that rather than seeking to provide anaverage lattice constant that matches the substrate, what is trulyimportant is to balance the forces in the tensile and compressivelystrained layer to provide an average or effective zero stress system. Adevice providing an average lattice constant matching the substrate maystill allow a significant build up of stress that will result inundesirable dislocations.

[0008] With this concept one can extend the absorption threshold tolonger wavelength without introducing dislocations.

[0009] With a strain-balanced multi-quantum-well stack in the intrinsicregion of a two-terminal photovoltaic device the absorption thresholdcan be extended to longer wavelengths. In particular, with high bandgapbarriers the dark current can be reduced at the same time, and hence theconversion efficiency is increased significantly.

[0010] What is also helpful to achieve higher conversion efficiencies isan improved voltage performance, due to a lower dark current. This isprovided by the higher barriers which may also be provided whenbalancing the strain.

[0011] Viewed from another aspect the invention provides a photovoltaicdevice having a multiple well quantum portion formed upon a virtualsubstrate having a virtual substrate lattice constant different than asubstrate lattice constant of an underlying substrate, wherein saidvirtual substrate is InP_(1−x)As_(x), where 0<x<1 and said substrate isInP.

[0012] Using an InP_(1−x)As_(x) based virtual substrate allows latticematching to a quantum well system having a relatively large latticeconstant, and typically desirable lowbandgap.

[0013] Viewed from a further aspect the invention provides aphotovoltaic device having a multiple quantum well portion formed ofstrained alternating quantum well layers of In_(x)Ga_(1−x)As, wherex>0.53, and barrier layers of Ga_(y)In_(1−y)P, where y>0.

[0014] This combination of layers allows provision of an advantageouslyhigh barrier energy within the multiple quantum well system whichreduces the dark current. Furthermore, this composition is well suitedto stress balancing and use with the above mentioned virtual substrate.

[0015] The above, and other objects, features and advantages of thisinvention will be apparent from the following detailed description ofillustrative embodiments which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a bandgap diagram of a strain-balanced quantum wellcell. The p- and n-regions are made of material that is lattice-matchedto the InP substrate, e.g. In_(0.53)Ga_(0.47)As or InP. The quantumwells are made of In_(x)Ga_(1−x)As with x >0.53, and the barrier ofIn_(x)Ga_(1−x)As with x<0.53, GaInP or InGaAsP;

[0017]FIG. 2 is a schematic drawing of a strain-compensated quantum wellcell where the width indicates the lattice parameter of the materialwhen unstrained;

[0018]FIG. 3 is a graph of dark current densities of a strain-balancedquantum well cell (as depicted in FIG. 2 but with 30 quantum wells)compared with bulk GaSb of similar effective bandgap (see FIG. 4) andlattice-matched bulk InGaAs;

[0019]FIG. 4 is a graph of modelled internal quantum efficiency (withback-surface reflector) of a strain-balanced quantum well cell (asdepicted in FIG. 2 but with 30 quantum wells) compared with bulk GaSband lattice-matched bulk InGaAs;

[0020]FIG. 5 is a graph of modelled internal quantum efficiency (withback-surface reflector) of a strain-balanced quantum well cell optimisedfor a Holmia emitter (not to scale);

[0021]FIG. 6 is a graph of the dark current of an AlGaAs/GaAs quantumwell cell, where the data (dots) is fitted (black line). The modelleddark current density for a QWC with a higher band-gap barrier (greyline) is reduced; and

[0022]FIG. 7 shows Lattice constant vs Bandgap of the material systemIn_(x)Ga_(1−x)As_(1−y)P_(y).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] A photovoltaic cell to convert low energy photons is described,consisting of a p-i-n diode with a strain-balanced multi-quantum-wellsystem incorporated in the intrinsic region. The bandgap of the quantumwells is lower than that of the lattice-matched material, while thebarriers have a much higher bandgap. The high band-gap barriers reducethe dark current. Hence the absorption can be extended to longerwavelengths, while maintaining a low dark current. This leads to greatlyimproved conversion efficiencies, particularly for low energy photonsfrom low temperature sources. This can be achieved by strain-balancingthe quantum wells and barriers, where each individual layer is below thecritical thickness and the strain is compensated by quantum wells andbarriers being strained in opposite directions. The strain iscompensated by choosing the material compositions and thicknesses of thelayers in such a way that the average stress is zero, taking intoaccount the elastic properties of the materials. Thereby the creation ofmisfit dislocations, which are detrimental to the dark current and henceto the cell conversion efficiency, can be avoided. The number of quantumwells that can be incorporated is therefore not limited by the build-upof strain, but only by the size of the i-region, and is typically 30-60[This is an important advantage over Freundlich's strained QWs with amaximum number of about 20 (see U.S. Pat No. 5,851,310 and U.S. Pat. No.6,150,604)]. The width of the i-region is limited by the electric fieldthat needs to be maintained across it.

[0024] The absorption can be further extended to longer wavelengths byintroducing a strain-relaxed layer (virtual substrate) between thesubstrate and the active cell. The device is then grown on this virtualsubstrate and the layers are strain-balanced with respect to the newlattice constant. This allows one to effectively move to a specificlattice constant which is associated with a desired band gap for thelattice matched and strain-balanced materials. This is of particularinterest for thermophotovoltaic applications with lower temperaturesources, as one can extend the absorption towards the required longwavelengths.

Description of the Preferred Embodiments

[0025] As an example for a strain-compensated QWC, we consider a 30 wellIn_(0.62)Ga_(0.38)As/In_(0.47)Ga_(0.53)As (InP) QWC, grown by MOVPE,whose sample description is given in Table I. TABLE I Sample descriptionof a strain-compensated quantum well cell. Conc. Layers Thickness (Å)Material Function Doping (cm⁻³) 1 1000 In_(0.53)Ga_(0.47)As Cap p 1E +19 1 7000 InP Emitter p 2E + 18 30 120 In_(0.45)Ga_(0.55)As Barrier i 30120 In_(0.62)Ga_(0.38)As Well i 1 120 In_(0.47)Ga_(0.53)As Barrier i 15000 InP Base n 1E + 18 InP Substrate n

[0026] In FIG. 2 the strain-balancing conditions of one example areshown, where the average lattice-constant of wells and barriers isroughly the same as the InP substrate. FIG. 1 shows a schematic diagramof the energy band-gaps of this kind of structure. This specific samplewas not designed for TPV applications; the p-region, for example, is fartoo thick. It does not quite fulfil the ideal strain-balancedconditions, but is close enough to avoid strain relaxation as is evidentby the low dark current of the device (see FIG. 3). In fact, the darkcurrent density is even lower than in a very good lattice-matched bulkInGaAs/InP cell [N. S. Fatemi et al., in Proc. 26th IEEE PV specialistsconf. (IEEE, USA, 1997), pp.799-804] as shown in FIG. 3. In FIG. 4 weshow the spectral response (SR) (=external quantum efficiency) data ofthe strain-balanced QWC at zero bias. The effective band-gap, resultingfrom the material composition and the confinement, is about 1.77 μm,which is well beyond the band-edge of lattice-matched InGaAs. Hence thestrain-balanced approach has enabled the absorption threshold to beextended out to 1.77 μm while retaining a dark current more appropriateto a cell with a band-edge of less than 1.65 μm. The band-edge of thestrain-balanced QWC is similar to that of a GaSb cell, but it has alower dark current (see FIG. 3). Strain-balanced QWCs in InGaP/InGaAs onGaAs have demonstrated dark currents comparable to homogenous GaAs cells[N. J. Ekins-Daukes et al., Appl.Phys.Lett.75, 4195 (1999)]. We haveshown (see FIG. 3) that, if anything, In_(x)Ga_(1−x)As/In_(z)Ga_(1−z)As(InP) cells with absorption edges out to 1.77 μm have lower darkcurrents than bulk InGaAs cells. To obtain even lower dark currents, weneed a higher band-gap in the barriers. We can achieve that by using adifferent material for the barrier, such as In_(x)Ga_(1−x)As_(1−y)P_(y)with y>0 or GaInP as indicated in FIG. 1, and an example for such adevice is given in Table II. TABLE II Sample description of astrain-balanced quantum well cell with high bandgap barriers. Conc.Layers Thickness (Å) Material Function Doping (cm⁻³) 1 1000In_(0.53)Ga_(0.47)As Cap p 1E + 19 1 1500 InP Emitter p 5E + 18 1 50Ga_(0.18)In_(0.82)P Barrier i 49 100 Ga_(0.18)In_(0.82)P Barrier i 50100 In_(0.72)Ga_(0.28)As Well i 1 50 Ga_(0.18)In_(0.82)P Barrier i 15000 InP Base n 1E + 18 InP Substrate n

[0027] We have developed a model which calculates the SR of multi-layerIn_(x)Ga_(1−x)As_(1−y)P_(y) devices, lattice-matched to InP (x=0.47 y)[M. Paxman et al., J.Appl.Phys.74, 614 (1993), C. Rohr et al., inThermophotovoltaic Generation of Electricity: Fourth NREL Conf., Vol.460of AIP Conf. Proc. (American Institute of Physics, Woodbury, N.Y.,1999), pp.83-92], which has been extended to estimate the SR ofstrain-balanced In_(x)Ga_(1−x)As/In_(z)Ga_(1−z)As on InP [C. Rohr etal., in Proc. 26th International Symposium on Compound SemiconductorsNo.166 in Institute of Physics Conference Series (Institute of PhysicsPublishing, Bristol and Philadelphia, 2000), pp.423-426]. The cellefficiency can be determined given the measured dark current data of thecell, assuming superposition of dark and light current. For photovoltaicapplications the p-region of a device would typically be as thin as 1500Å (instead of 7000 Å) in order to increase the light level that reachesthe active i-region where carrier separation is most efficient and toreduce free carrier absorption. A mirror on the back of asemi-insulating (i.e. charge neutral) substrate is particularly usefulfor QWCs as it enhances the well contribution significantly. The effectof such a mirror is simulated by doubling the light pass through thewells. The strain-balanced QWC is modelled with these modifications and,for the purpose of comparison, the reflectivity is removed to show theinternal quantum efficiency in FIG. 4.

[0028] We compare our strain-balanced QWC as well as our lattice-matchedInGaAsP QWCs with lattice-matched InGaAs monolithic interconnectedmodules (MIMs) [N. S. Fatemi et al., in Proc. 26th IEEE PV specialistsconf. (IEEE, USA, 1997), pp.799-804], one of the best lattice-matchedbulk InGaAs/InP TPV cells, and with bulk GaSb [A. W. Bett et al., inThermophotovoltaic Generation of Electricity: Third NREL Conf., Vol.401of AIP Conf. Proc. (American Institute of Physics, Woodbury, N.Y.,1997), pp. 41-53], currently the only material which is being usedcommercially for TPV applications. To compare efficiencies we assume‘typical’ TPV conditions of 100 kW/m² normalised power density, gridshading of 5%, and internal quantum efficiencies for all cells. A backsurface reflector is an integral part of MIM technology and particularlyuseful for QWCs as it enhances the well contribution significantly. Italso increases TPV system efficiency because longer wavelengthradiation, that is not absorbed by the cell, is reflected back to thesource. The efficiency projections for various illuminating spectra arecalculated from data presented in FIGS. 3 and 4 and are summarised inTable III. The relative efficiencies are rather more reliable than theabsolute values. TABLE III Comparison of predicted efficiencies (in %)of bulk InGaAs MIM, GaSb, lattice-matched and strain-balanced quantumwell cells with back-mirror using internal quantum efficiencies, undervarious spectra at 100 kW/m², and 5% grid shading: Bulk InGaAs InGaAsPStrain-bal. Spectrum MIM Bulk GaSb QWC QWC Solar × 100 16 16 20 19 3200K18 18 22 27 blackbody 2000K 11 11 12 22 blackbody 1500K 5.5 5.6 4.8 14blackbody MgO 13 15 16 41 Ytterbia 26 25 42 32 Erbia 37 37 46 43 Holmia4.5 5.4 4.1 39

[0029] The lower dark current of the QWCs (see FIG. 3) is the mainreason for their higher efficiencies in Table III. The lattice-matchedInGaAsP QWC shows higher efficiencies than the InGaAs MIM and GaSb inall cases except for black-body temperatures below about 2000 K. Higherblack-body temperatures, for example 3200 K and the solar spectrum AM1.5(approximating 5800 K) at 100 times concentration, are favourable forthe lattice-matched InGaAsP QWC. At black-body temperatures around 2000K and below, the strain-balanced QWC outperforms the others.Particularly with the MgO emitter, which was designed for a GaSb cell[L. Ferguson and L. Fraas, in Thermophotovoltaic Generation ofElectricity: Third NREL Conference Vol.401 of AIP Conf. Proc. (AmericanInstitute of Physics, Woodbury, N.Y., 1997), pp. 169-179], thestrain-balanced QWC is significantly better and shows an efficiencywhich is about 50% higher than that of a GaSb cell (see Table III).

[0030] Based on these results it should be possible to use this conceptof strain-balanced QWCs to extend the absorption threshold even further,beyond 2 μm, optimised for TPV applications with a Holmia emitter (seeFIG. 5). The efficiency for such a strain-balanced QWC with a Holmiaemitter [M. F. Rose et al., Journal of Propulsion and Power 12, 83(1996)] is predicted to reach 39% under the same conditions as discussedabove. The more the band-edge of a PV cell is extended towards longerwavelengths, the more suitable it becomes for lower temperature sources.

[0031] The conversion efficiency can be further substantially increasedby reducing the dark current. In strain-balanced devices, this can beachieved if higher band-gap material is used for the barriers asindicated in FIG. 1 and Table II.

[0032] A model for the dark current behaviour of QWCs is used in FIG. 6.In FIG. 6, a dark current density of an AlGaAs/GaAs quantum well cell isfitted, and it shows that the modelled dark current density for a QWCwith a higher band-gap barrier is reduced and hence the efficiency willbe increased.

[0033] In order to be lattice-matched to an InP substrate, the materialcomposition of In_(x)Ga_(1−x)As_(1−y)P_(y) must be chosen to lie on thevertical line in FIG. 7 going through InP, which corresponds tox≈0.53+0.47 y. That means, the lowest bandgap one can achieve withlattice-matched material is with In_(0.53)Ga_(0.47)As, a bandgap ofE_(g)≈0.74 eV. Strain-compensation in a multi-layer system allows one toachieve lower effective band-gaps. The quantum wells are compressivelystrain, going down the branch from In_(0.53)Ga_(0.47)As towards InAs(i.e. x>0.53), and to compensate the barriers have tensile strain goingup the branch from In_(0.53)Ga_(0.47)As towards GaAs (i.e. x<0.53). Toimprove the dark current with higher bandgap barriers one can usematerial compositions with y>0 and the same lattice constant as before,i.e. going up on a vertical line in FIG. 7. To achieve high bandgapbarriers, these may be formed of Ga_(y)In_(1−y)P, where y>0. In FIG. 7this composition follows the upper limit between InP and GaP.

[0034] By introducing a virtual substrate, still lower bandgaps can bereached as the lattice constant is increased by relaxed buffer layers.This shifts the base or reference line for strain-compensation towardsthe right in FIG. 7. This virtual substrate can be made of InAsP (upperbranch in FIG. 7) [Wilt et al., 28th IEEE PVSC (2000), p. 1024] insteadof InGaAs. Such an InAsP buffer is better in confining the dislocationsin the virtual substrate, which is crucial for successfully growing astrain-compensated multi-quantum well (MQW) structure on top of it.

[0035] The conditions for zero-stress strain-balance may be determinedfrom the following considerations:

[0036] The strain ε for each layer i is defined as$ɛ_{i} = \frac{a_{0} - a_{i}}{a_{i}}$

[0037] where a₀ is the lattice constant of the substrate (or virtualsubstrate), and a₁ is the natural unstrained lattice constant of layeri.

[0038] A strain-balanced structure should be designed such that a singleperiod composed of one tensile and one compressively strained layer,exerts no shear force on its neighbouring layers or substrate. Toachieve such a zero stress situation, one needs to taken into accountthe differences in elastic properties of the layers. Applying linearelastic theory one can derive the following conditions

[0039] e₁t₁A₁a₂+e₂t₂A₂a₁=0 (zero-stress condition)$a_{0} = {\frac{{t_{1}A_{1}a_{1}a_{2}^{2}} + {t_{2}A_{2}a_{2}a_{1}^{2}}}{{t_{1}A_{1}a_{2}^{2}} + {t_{2}A_{2}a_{1}^{2}}}\quad 5\quad \left( {{Match}\quad {substrate}\quad {lattice}\quad {constant}} \right)}$$A = {C_{11} + C_{12} - {\frac{2C_{12}^{2}}{C_{11}}\quad \left( {{Layer}\quad {stiffness}} \right)}}$

[0040] where t₁ and t₂ are the thicknesses of layers 1 and 2, and C₁₁and C₁₂ are the elastic stiffness coefficients.

[0041] Although illustrative embodiments of the invention have beendescribed in detail herein with reference to the accompanying drawings,it is to be understood that the invention is not limited to thoseprecise embodiments, and that various changes and modifications can beeffected therein by one skilled in the art without departing from thescope and spirit of the invention as defined by the appended claims.

We claim:
 1. A photovoltaic device having a multiple quantum wellportion with alternating tensile strained layers and compressivelystrained layers, said tensile strained layers and said compressivelystrained layers having compositions such that a period of one tensilestrained layer and one compressively strained layer exerts substantiallyno shear force on a neighbouring structure.
 2. A photovoltaic device asclaimed in claim 1, wherein said neighbouring structure is one of: afurther period of one tensile strained layer and one compressivelystrained layer; a layer of arbitrary doping having the same latticeconstant as a substrate; and a substrate.
 3. A photovoltaic device asclaimed in claim 1, being a crystalline photovoltaic device grown upon asubstrate with a substrate lattice constant.
 4. A photovoltaic device asclaimed in claim 3, wherein at least one of said tensile strained layersor said compressively strained layers is a quantum well having a GroupIII/V semiconductor composition with a bandgap lower than if saidquantum well had a lattice constant equal to said substrate latticeconstant.
 5. A photovoltaic device as claimed in claim 3, wherein atleast one of said tensile strained layers or said compressively strainedlayers is a barrier having a Group III/V semiconductor composition witha bandgap higher than if said barrier had a lattice constant equal tosaid substrate lattice constant.
 6. A photovoltaic device as claimed inclaim 1, wherein said multiple quantum well portion is formed ofalternating quantum well layers and barrier layers having a Group III/Vsemiconductor composition, wherein a period of one quantum well layerand one quantum barrier layer contains at least four different elementsand has an average lattice constant substantially matching aneighbouring structure lattice constant.
 7. A photovoltaic device asclaimed in claim 4, wherein said substrate is InP and said compressivelystrained layer is In_(x)Ga_(1−x)As, where x>0.53.
 8. A photovoltaicdevice as claimed in claim 5, wherein said substrate is InP and saidtensile strained layer is In_(x)Ga_(1−x)As_(1−y)P_(y), where y>0.
 9. Aphotovoltaic device as claimed in claim 8, wherein y=1 such that saidtensile strained layer is GaInP.
 10. A photovoltaic device as claimed inclaim 3, wherein said substrate is InP and said multiple quantum wellportion is formed of layers of Al_(x)Ga_(1−x)As_(y)Sb_(1−y), where 0≦x≦1and 0≦y≦1.
 11. A photovoltaic device as claimed in claim 3, wherein saidsubstrate is GaSb and said multiple quantum well portion is formed oflayers of In_(x)Ga_(1−x)As_(y)Sb_(1−y), where 0≦x≦1 and 0≦y≦1.
 12. Aphotovoltaic device as claimed in claim 3, wherein said substrate isGaAs.
 13. A photovoltaic device as claimed in claim 12, wherein saidmultiple quantum well portion is formed of layers ofIn_(x)Ga_(1−x)As_(y)P_(1−y), where 0≦x≦1 and 0≦y≦1.
 14. A photovoltaicdevice as claimed in claim 1, wherein said multiple quantum well portionis formed upon a virtual substrate composed of a strain relaxed bufferlayer having a virtual substrate lattice constant different from asubstrate lattice constant of an underlying substrate.
 15. Aphotovoltaic device as claimed in claim 14, wherein said virtualsubstrate is InP_(1−y)As_(y), where 0<y<1, and said substrate is InP.16. A photovoltaic device as claimed in claim 1, wherein saidphotovoltaic device is a thermophotovoltaic device.
 17. A photovoltaicdevice as claimed in claim 1, wherein said quantum wells have a bandgapsubstantially equal to or less than 0.73 eV
 18. A photovoltaic devicehaving a multiple well quantum portion formed upon a virtual substratehaving a virtual substrate lattice constant different than a substratelattice constant of an underlying substrate, wherein said virtualsubstrate is InP_(1−x)As_(x), where 0<x<1, and said substrate is InP.19. A photovoltaic device as claimed in claim 18, wherein said multiplequantum well portion is formed with alternating tensile strained layersand compressively strained layers, said tensile strained layers and saidcompressively strained layers having compositions such that a period ofone tensile strained layer and one compressively strained layer exertssubstantially no shear force on a neighbouring structure.
 20. Aphotovoltaic device as claimed in claim 19, wherein said neighbouringstructure is one of: a further period of one tensile strained layer andone compressively strained layer; a layer of arbitrary doping having thesame lattice constant as said virtual substrate; and said virtualsubstrate.
 21. A photovoltaic device as claimed in claim 18, wherein atleast one of said tensile strained layers or said compressively strainedlayers is a quantum well having a Group III/V semiconductor compositionwith a bandgap lower than if said quantum well had a lattice constantequal to said substrate lattice constant.
 22. A photovoltaic device asclaimed in claim 18, wherein at least one of said tensile strainedlayers or said compressively strained layers is a barrier having a GroupIII/V semiconductor composition with a bandgap higher than if saidbarrier had a lattice constant equal to said substrate lattice constant.23. A photovoltaic device as claimed in claim 18, wherein said multiplequantum well portion is formed of alternating quantum well layers andbarrier layers having a Group III/V semiconductor composition, wherein aperiod of one quantum well layer and one quantum barrier layer containsat least four different elements and has an average lattice constantsubstantially matching a neighbouring structure lattice constant.
 24. Aphotovoltaic device as claimed in claim 21, wherein said substrate isInP and said compressively strained layer is In_(x)Ga_(1−x)As, where xis larger than z of In_(z)Ga_(1−z)As which is lattice-matched to thevirtual substrate.
 25. A photovoltaic device as claimed in claim 23,wherein said substrate is InP and said tensile strained layer isIn_(x)Ga_(1−x)As_(1−y)P_(y), where y>0.
 26. A photovoltaic device asclaimed in claim 25, wherein y=1 such that said tensile strained layeris GaInP or wherein x=1 such that said tensile strained layer is InAsP27. A photovoltaic device as claimed in claim 18, wherein said substrateis InP and said multiple quantum well portion is formed of layers ofAl_(x)Ga_(1−x)As_(y)Sb_(1−y), where 0≦x≦1 and 0≦y≦1.
 28. A photovoltaicdevice as claimed in claim 18, wherein said substrate is GaSb and saidmultiple quantum well portion is formed of layers ofIn_(x)Ga_(1−x)As_(y)Sb_(1−y), where 0≦x≦1 and 0≦y≦1.
 29. A photovoltaicdevice as claimed in claim 18, wherein said substrate is GaAs.
 30. Aphotovoltaic device as claimed in claim 29, wherein said multiplequantum well portion is formed of layers of In_(x)Ga_(1−x)As_(y)P_(1−y),where 0<x<1 and 0<y<1.
 31. A photovoltaic device as claimed in claim 18,wherein said photovoltaic device is a thermophotovoltaic device.
 32. Aphotovoltaic device as claimed in claim 18, wherein said quantum wellshave a bandgap substantially equal to or less than 0.73 eV
 33. Aphotovoltaic device having a multiple quantum well portion formed ofstrained alternating quantum well layers of In_(x),Ga_(1−x)As, wherex>0.53, and barrier layers of Ga_(y)In_(1−y)P, where y>0.
 34. Aphotovoltaic device as claimed in claim 33, wherein said multiplequantum well portion is formed with alternating tensile strained layersand compressively strained layers, said tensile strained layers and saidcompressively strained layers having compositions such that a period ofone tensile strained layer and one compressively strained layer exertssubstantially no shear force on a neighbouring structure.
 35. Aphotovoltaic device as claimed in claim 34, wherein said neighbouringstructure is one of: a further period of one tensile strained layer andone compressively strained layer; a layer of arbitrary doping having thesame lattice constant as a substrate; and a substrate.
 36. Aphotovoltaic device as claimed in claim 33, being a crystallinephotovoltaic device grown upon a substrate layer with a substratelattice constant.
 37. A photovoltaic device as claimed in claim 36,wherein said substrate is InP.
 38. A photovoltaic device as claimed inclaim 33, wherein said multiple quantum well portion is formed upon avirtual substrate composed of a strain relaxed buffer layer having avirtual substrate lattice constant different from a substrate latticeconstant of an underlying substrate.
 39. A photovoltaic device asclaimed in claim 38, wherein said virtual substrate is InP_(1−y)As_(y),where 0<y<1, and said substrate is InP.
 40. A photovoltaic device asclaimed in claim 33, wherein said photovoltaic device is athermophotovoltaic device.
 41. A photovoltaic device as claimed in claim33, wherein said quantum wells have a bandgap substantially equal to orless than 0.73 eV.